Many industrial processes require the use of tubing which exhibits different qualities between the inside surface and the outside surface thereof, such as corrosion resistance. For such applications, the use of multi-component metallic tubing can be desirable, to provide the desired combination of corrosion-resistance and strength characteristics for the specific application. For such applications, corrosion-resistance and resistance to delamination are desired, particularly attendant to thermal cycling and/or heat transfer of the composite structure. Such materials find applications in boiler tubes for power generation, oil and gas processing, waste water handling, desalination processes, and a wide range of similar applications where the multi-component nature of the tubing can be advantageously employed. U.S. Pat. No. 5,190,832; U.S. Pat. No. 5,324,595; U.S. Pat. No. 5,620,805; U.S. Pat. No. 5,940,951; U.S. Pat. No. 6,303,237; and U.S. Pat. No. 6,865,920, all hereby incorporated by reference, relate to composite tubular structures and their formation.
Heretofore, efforts to make such bi-metallic tubing have been limited. It is believed that the use of sintering processes are known, wherein a powered metal starting billet is canned, sintered, and hipped to produce a starting billet. This billet is then extruded to form a bi-metallic tubular structure. For example, see U.S. Pat. No. 5,056,209, hereby incorporated by reference.
The so-called weld overlay process is also quite common, and an accepted method to produce a nickel-based alloy metallurgically bonded to the exterior surface of a tube. This process is used frequently in connection with new boiler panel installations, and can also be done on-site to repair existing boiler panel installations.
It is recognized that both of these techniques known heretofore suffer from specific disadvantages. In connection with the extrusion process using a powdered metal or canned billet when producing carbon steel tubes with a stainless steel overlay, it is known that stainless steel alloys are inherently not as corrosion-resistant as nickel-based alloys. The bond strength between the two materials may be weak, which can have detrimental performance implications, particularly in those environments where thermal cycling is present. By the use of a canned billet, the metallurgical bond formed between the core material and the overlay results from the extrusion process, rather than from the billet forming process. Additionally, the production of a powdered or sintered billet can be labor-intensive and costly.
The weld overlay process also suffers from distinct disadvantages. Uneven surface thickness uniformity can create heat transfer problems, and undesirable residual stresses within the tubular structure. The welding process itself typically creates residual stresses. The weld overlay process is difficult to apply on existing installations, and is very costly to apply in both new and existing installations. Labor costs to effect the weld overlay process are expensive, and additionally, this process must be performed on-site by qualified professionals. In the case of a coal-fired power plant installation, the down time cost for the installation can be very significant.
The present invention is directed to a method of forming a multi-component, composite metallic tube, such as a bi-metallic tube for applications in power-generation, and other applications which require corrosion-resistant performance, and resistance to delamination attended to thermal cycling. Notably, the present method achieves a metallurgical bond within the multi-component structure to provide the necessary performance characteristics in a highly cost-effective fashion.